Flow measurement and control take an important part in the field of industry. Currently, flow controllers on the market for controlling the passing flow of a device comprise gas flow controller and fluid flow controller. As shown in FIG. 1, in a flow controller, the flow signal measured by a sensor, which is usually an analog voltage signal or an analog current signal, is converted to be a digital signal by an A/D converter and transmitted to a microcontroller. After calculation, the microcontroller outputs a digital control signal to a D/A module, which converts the digital control signal to an analog voltage. Then, the valve control unit converts the analog voltage to an electrical signal to control the mechanical valve. The mechanical valve can be of any structure such as an electromagnetic valve, a thermal valve or a Piezoelectric valve and so on, which realizes different work modes according to the voltage or the current of the electrical signal. In other words, the flow control can be realized by controlling the electrical signal. As shown in FIG. 2, the conventional valve control circuit comprises at least one D/A module performing the function of digital value to analog voltage conversion; one or more operational amplifier U2 performing amplifying and gain adjusting; and one or more transistors of high power Q1, Q2.
However, the conventional valve control circuit has the following disadvantages: since the valve control circuit requires high precision for the D/A module and large power for the transistors, the power loss of the whole product may be excessively high. Furthermore, the heat radiation from the transistors may also cause the change of the environmental temperature, which may reduce the measurement accuracy of the circuit as well as the service life of the components.